1. Field of the Invention
The present invention relates to a nitride semiconductor laser element, particularly a nitride semiconductor laser element having a high COD (catastrophic optical damage) level.
2. Description of the Background Art
Nitride semiconductor laser elements are promising in the field of application to the light source of short wavelength light for reading and writing optical disk information as well as the light source of short wavelength light of high power.
For the active layer of a nitride semiconductor laser element, research efforts are made on a quantum well active layer of a quantum well structure having a well layer and a barrier layer stacked. In a quantum well active layer, the state density function ρof electrons and holes can be regulated narrow artificially. Therefore, the usage of a quantum well active layer allows the properties of a nitride semiconductor laser element to be improved since the oscillation threshold value of the nitride semiconductor laser element can be reduced than in the case where an active layer that does not have a quantum well structure (such as a nitride semiconductor laser element of a double hetero structure type) is employed.
For example, Patent Document 1 (Japanese Patent Laying-Open No. 2004-031770) discloses a nitride semiconductor light emitting element having a barrier layer formed of a plurality of layers including a doped layer into which n type impurities are doped and an undoped layer into which n type impurities are not doped, wherein the doped layer of the barrier layer is arranged adjacent to the well layer.
This nitride semiconductor light emitting element aims to facilitate generation of carriers efficiently while suppressing the concentration of n impurities in the barrier layer and suppressing internal loss caused by free carrier dispersion. This nitride semiconductor light emitting element is based on the approach of generating carriers by doping n type impurities into the active layer to reduce the threshold current at the time of driving.
Patent Document 2 (Japanese Patent Laying-Open No. 10-084132) discloses a nitride semiconductor light emitting diode element employing undoped GaN for the barrier layer and undoped In0.2Ga0.8N for the well layer with a buffer layer of undoped In0.1Ga0.9N inserted between the well layer and barrier layer.
In the nitride semiconductor light emitting diode element of Patent Document 2, the buffer layer is provided for the purpose of alleviating variation in the lattice constant between the well layer and the barrier layer. A nitride semiconductor having a high In composition ratio of 0.2 is employed for the well layer. Furthermore, GaN is employed for the barrier layer to provide a large potential barrier between the well layer and the barrier layer to increase light emission.
Therefore, the difference in lattice constant between the well layer and the barrier layer is regarded as a problem in the nitride semiconductor light emitting diode element of Patent Document 2. In order to alleviate this difference in lattice constant, it is necessary to employ a nitride semiconductor having a high In composition ratio of 0.1.